Impact of Entanglement on Folding of Semicrystalline Polymer during Crystallization

Upon cooling, semicrystalline polymers experience crystallization and form alternatively stacked layers consisting of thin crystal lamellae and amorphous ones. The unique morphology, crystallinity, and crystallization kinetics highly depend on the molecular weight. Therefore, it is deduced that entanglement impacts crystallization kinetics, as well as hierarchically crystalline structures. However, the impact of entanglement on folded crystalline chains has not been well understood due to experimental difficulties. In this work, chain-folding structures for seven ¹³C CH₃ labeled poly(l-lactic acid)s with various molecular weights (M_ws) were investigated by ¹³C-¹³C double quantum NMR spectroscopy. As a result, chain-folding events were categorized into three different M_w regimes: (i) The lowest M_w sample (2K g/mol) adopts an extended chain conformation (folding number, n = 0) (regime I); (ii) Intermediate M_w ones possess mixtures of non- and once-folded structures, and the once-folded fraction suddenly increases above the entanglement length (M_e), up to M_w = 45K g/mol (regime II); (iii) The high M_w ones (M_w > 45K g/mol) adopt the highest chance for an adjacent re-entry structure with n = 1.0 in the well-developed entangled network (regime III). It was suggested that entanglement induces folding of the semicrystalline polymer.